The invention concerns a method for controlling the transfer of magnetization within an interacting spin system using nuclear magnetic resonance (NMR) in which the spin system is subjected to a homogeneous magnetic field and comprises a source spin component, a target spin component, and one or several perturbing spin components, the method comprising irradiating a first radio frequency (RF) pulse to selectively invert the longitudinal magnetization of the source spin component along a direction of the homogeneous magnetic field and subsequently, at a time .tau..sub.m / 2 following the first pulse, irradiating a second selectively interacting RF pulse to control the transfer of magnetization from the source spin component to the perturbing spin component and irradiating, at a mixing time .tau..sub.m following the first pulse, a third RF pulse to convert a resulting longitudinal magnetization into a transverse magnetization. A method of this type is known in studies of rate determination of chemical reactions for systems in dynamic equilibrium, where the turnover due to forward and backward reactions precisely cancel each other, so that there is no net transfer of material.sup.1. In most forms of spectroscopy, there is little or no evidence that any dynamic processes are taking place, since the concentrations remain time-independent, but magnetic resonance allows one to observe the transfer of magnetization rather than the transfer of material, and it is sufficient that the chemical shifts of the nuclei be affected in the course of the chemical reaction to make the exchange apparent. Apart from line-shape studies.sup.2,3, this effect can be exploited in experiments where the longitudinal magnetization of a chosen site is perturbed by selective saturation or inversion.sup.4-7 The exchange processes lead to a distribution of this perturbation in the course of a reaction or "mixing" time .tau..sub.m. This effect can be visualized very effectively by two-dimensional exchange spectroscopy (sometimes called EXSY), which is, in many respects, similar to the Nuclear Overhauser Effect Spectroscopy (NOESY).sup.8,9.
These methods have the disadvantage that in addition to interacting with the spin system of which exchange is to be studied, magnetization also interacts with one or several other spin systems, the so-called perturbing spin system, as well. This additional interaction obscures the characterization of the exchange reaction of the desired spin system and renders analysis of the experiments more difficult.
Similar problems are encountered in Nuclear Overhauser Effect Spectroscopy (NOESY) .sup.8,9. In these experiments of high resolution nuclear magnetic resonance (NMR) the transfer of longitudinal magnetization from one spin site to another under the effect of cross relaxation (Nuclear Overhauser Effect) is also complicated by spin diffusion pathways through other spins in the vicinity. In this connection a method called "Quenching of Undesirable Indirect External Trouble in Nuclear Overhauser Effect Spectroscopy" (QUIET-NOESY) has been developed.sup.11 to quench these undesired pathways through manipulation of the magnetization of the two sites by means of doubly selective inversion pulses. The experiment is begun through selective inversion of a source spin. The longitudinal magnetization, thereby, tends to migrate not only to a target nuclei but also to various other perturbing nuclei. The migration is controlled by simultaneously inverting the longitudinal magnetization components of both the source and the target spins in the middle of the mixing interval .tau..sub.m without affecting those of the perturbing spin. In this fashion the direct flow of magnetization from the source to the target spin is not perturbed but the indirect flow to the perturbing spins is reversed in sign and almost perfectly cancelled at the end of the relaxation interval .tau..sub.m.
It is the purpose of the present invention to develop a method for inhibiting the transfer of magnetization in chemically exchanging systems with several sites in dynamic equilibrium so that the forward- and backward reaction rates involving two selected species can be studied without being significantly perturbed by other exchange processes.